Make distance solr search in miles in Drupal 8

If you need to do the solr distance search based on the city or the zipcode, latitude and longitude needs to be passed to the solr in query filter query (fq). With geofilt sfiled describes the indexed field which holds the geo location coordinates.
In sfield solr field which contains the latitude+longitude (comma separated) information needs to be passed.
In this example locs_field_chapter_geofield holds such information.

In the hook_search_api_solr_query_alter with filter query (fq) geofilt  
and pt (latitude+longitude comma separate), d (radius in km or miles)
and score (type search km/miles) solr is hit for the distnace search.

function modulename_search_api_solr_query_alter(\Solarium\Core\Query\QueryInterface $solarium_query, \Drupal\search_api\Query\QueryInterface $query) {
  $zip = (isset($_REQUEST['zip'])) ? trim(strtolower($_REQUEST['zip'])) : '';
  $state = (isset($_REQUEST['state'])) ? trim(strtolower($_REQUEST['state'])) : '';
  $radius = (isset($_REQUEST['radius'])) ? $_REQUEST['radius'] : 5;
  $radius_type = (isset($_REQUEST['radius_type'])) ? $_REQUEST['radius_type'] : 'miles';
  $lat_lng = '';
  if ($zip) {
    $lat_lng =  modulename_get_latitue_longitude($zip);
  }
  if (empty($state) && !empty($lat_lng)) {
    $solarium_query->addParam('pt', $lat_lng);
    $solarium_query->addParam('d', $radius);
    $solarium_query->addParam('score', $radius_type);

    $solarium_query->addParam('fq', '{!geofilt sfield=locs_field_chapter_geofield}');
  }

  }

}

This function returns the geolocation latitude+longitude (comma separated) data.

function modulename_get_latitue_longitude($zip) {
  $geolocation = \Drupal::service('geolocation.core');
  $geocoderManager = $geolocation->getGeocoderManager();
  $geocoder = $geocoderManager->getGeocoder('google_geocoding_api');
  $geocode = $geocoder->geocode($zip);
  $lat_lng = '';
  if (isset($geocode['location'])) {
    $lat_lng = $geocode['location']['lat'] . ',' . $geocode['location']['lng'];
  }
  return $lat_lng;
}

Get latitude longitude in drupal 8 based on city or zipcode

Geolocation provides latitude and longitude information. In order to search based on these values, when zipcode or city name is passed to this function, data is returned comma separated.
$lat_lan = module_name_get_latitue_longitude(‘delhi’);
$lat_lan = module_name_get_latitue_longitude(‘110001’);

/**
 * Get Latitude longitude.
 *
 * @param type $zip
 *   Zipcode
 *
 * @return string
 *   Returns latitude and longitude, if found.
 */
function module_name_get_latitue_longitude($zip) {
  $geolocation = \Drupal::service('geolocation.core');
  $geocoderManager = $geolocation->getGeocoderManager();
  $geocoder = $geocoderManager->getGeocoder('google_geocoding_api');
  $geocode = $geocoder->geocode($zip);
  $lat_lng = '';
  if (isset($geocode['location'])) {
    $lat_lng = $geocode['location']['lat'] . ',' . $geocode['location']['lng'];
  }
  return $lat_lng;
}